1. Field of the Invention
The present invention relates to a method for production of a thin-film semiconductor device and, more particularly, to a method for production of a thin-film semiconductor device of the bottom gate type.
2. Description of the Related Art
Flat panel displays, such as liquid crystal displays and organic EL displays, have thin-film transistors (TFTs) as elements to drive pixel electrodes. Of these TFTs, poly-Si TFTs in which the semiconductor thin film constituting the active layer is made of poly-Si are attracting attention because they form driving circuits and permit highly functional circuits to be built into the panel, thereby giving the so-called system-on-glass structure. Attempts have been made to form poly-Si TFTs on a low-cost glass substrate through the so-called low-temperature poly-Si process which is carried out at 600° C. and under.
Production of poly-Si TFTs through the low-temperature poly-Si process is subject to contamination with impurities, such as metal ions, contained in the insulating substrate, such as glass. Such contamination is usually avoided by a silicon nitride film to exclude the diffusion of impurities, which is interposed between the substrate and the poly-Si film. Unfortunately, the poly-Si film in direct contact with the silicon nitride film deteriorates the characteristic properties of elements on account of fixed charges in the silicon nitride film and interface states originating from the interface of the poly-Si film. Common practice to cope with this situation is the addition of a silicon oxide film onto the silicon nitride film placed underneath the poly-Si film. (See Japanese Patent Laid-open No. 2003-124469)
However, the foregoing two-layer structure is not suitable for TFTs of the bottom gate type with gate electrodes formed underneath the poly-Si film as the active layer, in which the insulating film underneath the poly-Si film functions as the gate insulating film. The two-layered gate insulating film inevitably becomes thick, which is detrimental to high-performance TFTs of the bottom gate type.
One way proposed so far to address this problem is by the use of a silicon oxynitride (SiON) film as the gate insulating film or the underlying insulating film of the poly-Si film. The silicon oxynitride (SiON) film permits a larger ON current to flow for the same gate electrode because silicon oxynitride (SiON) has a higher relative permittivity than silicon oxide (SiO2). In addition, silicon oxynitride (SiON) also stops mobile ions, such as Na+, which contributes to the reliability of TFTs. (See Japanese Patent Laid-open No. 2003-209261) TFTs of the bottom gate type are produced in the following way. First, the substrate has gate electrodes formed thereon, the gate electrodes are covered with a gate insulating film of silicon oxynitride, and the gate insulating film is covered with a semiconductor thin film. Next, the semiconductor thin film is covered with a protective film of silicon oxide for protection of the interface at the back gate side. Finally, the semiconductor film is processed through the protective film.